Braking problems?

Has anyone tried to fasten the floor mat down with velcro or something????

Don't have my prius yet..but I'll be looking out for the floor mat issue!

thanks folks!

 

I'm actually not going that fast, it's a 35mph road. And none of my other cars with ABS have ever had the same problem. I just think the prius's ABS overcompensates, and not in a good way.

 

The floor mat (if OEM) has holes for hooks that are on the floor under the seat. That prevents it from sliding up.

 

making a hole cant be that tough

 

While this is a good explanation for aircraft, this is not how automotive ABS works. For aircraft, slip is controlled such that no wheel will ever lock for the sake of stability. Also, the nose wheel is not braked. For autos, the control algorithm uses all of the available speed information to calculate vehicle speed. This can be as few as 3 sensors in some applications (rear axle differential sensor only) but 4 in most cases as with the Prius. When any wheel exceeds the maximum allowable slip level for a given vehicle speed and deceleration, ABS will then become active. All brakes are not released if two or three or all four wheels lock. Rather, as individual wheels are released, the vehicle speed calculation is updated and brake torque is maintained to all wheels. In ABS, for most surfaces brake force is REDUCED, not eliminated, to a wheel. The reduction is based on vehicle speed, vehicle deceleration, and individual wheel deceleration. This allows for a surface coefficient estimation and further minimizes stopping distance. For transitions, brake force is generally fully restored within 1.5 seconds (worst case is ice to dry pavement transition). By design, the algorithm goes to great efforts to never release pressure at all brakes at the same time.

Back to the original problem. For the brake system used in the Prius, which is unique, brake apply pressure is a function of pedal travel and rate of pedal travel. Unless the system is in bypass (failure) mode, hydraulic brake pressure is metered to the wheels via solenoids. If the solenoids don't open, only regenerative braking is available and only if the brake is applied. So, to get "NO brakes" we either need a brake travel sensor failure, a solenoid control failure, a brake control computer problem, a bad wheel speed sensor (results in corrupted wheel speed calculation) or not enough brake travel to force brake torque generation. Assuming it is not a floor mat issue, and since there were no codes reported, and since stomping on the brake pedal as described should also have prompted brake assist to active, fully applying the brakes immediately, we are left with the pedal travel sensor or lack of pedal travel as the most likely causes. Of course, if the throttle was still depressed and/or the brake travel was obstructed, this would not be the case.

One item I would check is the pedal travel sensor and brake switch. Anyone remember the recall for the brake switches? Not sure if it applies to this vehicle based on the reported purchase date, but just prior to replacement I had two unexplainable incidents at low speed where I had no brakes for a few moments. One incident nearly resulted in contact and I was really pushing on the brake. Since I was informed about the brake switch recall at about the same time and had the work done less than a week later, I have not had anything like this reoccur. Does this vehicle fit within the recall VINs? Other than that, I would check over the repair receipts for any replaced brake related components and if none are found, I would call the dealer and ask if anything was done and not noted, such as the brake switch or some other unknown change. This brake system has a lot of redundancy, but if it does not work as intended, it is generally no easy task to figure out why.

 

I just got home from lab a few minutes ago and thought I'd share this. I took a different route home than usual because I dropped a friend off at one of the vastly remote parking lots they have here at Duke... and as I was slowing down to stop at a red light... bump! I hit a pothole and for a second there, the brakes did let off. But it wasn't a huge issue, I was still able to stop. I wasn't really going too fast anyway.

The brakes do have a backup power supply but I don't know what it takes for that to kick in. I imagine in a situation like this it probably isn't even relevant to think about using the backup power supply. I think that's just for whole-vehicle power failure.

 

I have experienced that a few times. It is just the ABS activating. Had you looked at the dashboard, you'd have seen the "skidding car" warning light.

 

DanMan - YUP! I'm getting OEM floor mats. So looks like I should have no problema.

thanks!

 

<div class='quotetop'>QUOTE(bramblerose11\";p=\"92022)</div>

Oh, come on now! Prius' don't accelerate THAT fast! :mrgreen:

 

<div class='quotetop'>QUOTE(IsrAmeriPrius\";p=\"92704)</div>

I have experienced that a few times. It is just the ABS activating. Had you looked at the dashboard, you'd have seen the "skidding car" warning light.[/b][/quote]

Shouldn't the ABS attempt to stop the car rather than letting it slide freely forward? The few times it's happened to me, it's like having no brakes on at all. And it's not how ABS works on other cars.

 

I think something that's missing in this discussion is the transition from regen braking to friction braking. THAT (and not the ABS per se) is what people are feeling when they describe the car lurching or slipping forward.

As I understand it, this IS one of the functions of the TC system. I may be wrong, but I think I have seen a TC light illuminate briefly during one of these "lurches".

The basic problem is that ABS doesn't work with regen braking. Before the ABS can do anything to resolve a developing "event", in fact before it can really decide whether or not it needs to do anything to resolve it, something has to happen first to transition the car from front-wheel regen braking to four-wheel friction braking.

During light braking, the slipping of one wheel due to rough road surface, skimming a pothole, or ramping up and off a bump can trigger the cancellation of regen braking and an immediate switch to friction braking.

This happens at a much lighter pedal pressure than would otherwise be the case, as during during a panic stop.

I hypothesize that since the pedal is already partially depressed, having simulated brake cylinder travel during regen, the friction brakes have to play catch-up, activating the master cylinder to match the brake pedal position Since that takes a moment, whereas cancellation of regen is virtually instantaneous, you can feel the transition as a slight "forward lurch".

All this happens before any actual ABS activation, and can happen for relatively trivial reasons that won't really result in an ABS activation at all. It appears to reset as soon as the brakes are released.

It's not enough to cause any real loss of control, but it is a counterintuitive seat-of-the-pants experience and can cause a definite Uh-Oh! reaction.

 

Brake system doesn't use the master cylinder brake pressure during normal operation. And by normal operation here, I mean that it is working, no malfunctions.

Take a look at the brake Q&A thread. There I posted the brake diagram.
When the brake system is operating properly, the brake system is 'by wire'. The brake pedal position sensor determines how much brake you are requesting. There is also a brake fluid pressure sensor in the master cylinder outputs, probably to detect panic stops as well as pressure failure. The stroke simulator is there to give you the feel of a traditional brake system, so that the pedal has some fluid resistance, otherwise it has no function to help you stop.

Now, once the skid (brake) ECU determines how fast you need to stop, it also checks all 4 wheel speed sensors to determine if ABS is needed. If not, then regen is used, and if regen is not enough, then hydraulic brakes. If you have VSC, then yaw sensor and steering wheel position sensor is checked to see if you are spinning.

 

<div class='quotetop'>QUOTE(DanMan32\";p=\"92891)</div>

For clarification, the pressure sensors allow closed loop brake pressure control, allowing for a given hydraulic pressure to be arrived at for a given pedal travel when not in ABS. This is controlled by the opening and closing of the solenoids. During ABS, if the pedal travel results in pressures high enough to lock the wheels, pressure is simply slip controlled, however if the wheel only locks occasionally, the sensors again allow the system to arrive at the target hydraulic pressure for a given pedal travel. Additionally, the sensors provide diagnostic feedback to allow detection of a mechanical solenoid failure. In that case, the electrical signal is valid, but the valve will either not open or close.

The stroke simulator does exactly as the name suggests. In by-wire mode, with the solenoids closed, there is no place for the brake fluid in the master cylinder to go since it has nearly zero compressibility. The stroke simulator provides both feedback (pedal resistance) and displacement for the fluid during a brake apply, else the pedal will not move, resulting in the travel sensor reading the same, eliminating the ability to make deceleration a function of brake pedal travel.

In the event of a brake system failure, the stroke simulator is bypassed, the solenoids are open, and the fluid displacement created by pressing on the brake pedal is what provides hydraulic pressure to the brakes. In other words, in failure mode, the brake system works as a conventional brake system.

Last, the back-up energy module is indeed for total system electrical failure. By design, the brake system will call for pressure any time the pedal is depressed. When the power is off, this energy is drawn from the module to avoid running down the 12v battery. Thus, if you were driving down the road and the car croaked totally, the brake system has both reserve (accumulator pressure) and energy module power to supply numerous full power brake applications. Because the sequence to power up the vehicle in READY mode requires a brake apply, not having the module could result in depletion of the accumulator pressure prior to power up and would also demand significant power when the 12 volt systems came alive, potentially resulting in transients and voltage drop, not good when you are trying to power up all the computers.

 

The pressure sensors I was referring to were the ones off of the master cylinder, which you correctly stated, is cut off from the main hydraulic system in 'drive by wire' mode. Therefore, the sensors would detect a panic stop request when you stomp on the brakes, especially when you hit 'bottom'.

By the way, I like that description: drive by wire mode. Clarifies alot.

 

I thought I'd revisit this post as yesterday the same thing happened to me.

I took a floater and went out to the hobby farm to drop off some groceries and supplies in preparation for a long weekend visit. The hobby farm is at the end of a 4km gravel sideroad.

The gravel sideroad is lightly traveled and rarely graded, so this time of the year it tends to be washboardy. Also, the terrain is rolling, the road has many dips and rises and curves, so the posted 50 km/h limit is prudent.

I was coasting down one gentle dip at 55 km/h when I noticed a deer eating the fresh grass along the road at the bottom. The deer gave that "I'm going to jump in front of you" look so I nailed the brakes. I had just hit another washboardy section of the road.

Nothing happened.

At least nothing that felt like a panic stop. It felt more like the gentle application to get regenerative "braking" instead. I didn't feel the ABS pulse either, though with the washboardy road I was bouncing over, it would be hard to tell.

I hit the horn and the deer went the other direction. Once past the washboardy section the car then braked strongly, though that was a good 25 ft beyond the point I originally applied the brakes. I would have brushed against the deer if she had stayed where she was.

I'm wondering if the poor suspension compliance of the Prius had anything to do with this? I've driven other vehicles with rock-hard buckboard ride quality and they also exhibit poor steering and braking over washboardy roads.

As an experiment, once at the hobby farm I took my old 1984 Ford F-150 for a ride out to the highway and back. In the 80's I put on a James Duff suspension kit, and several years ago I put on Rancho 9000 adjustable shocks.

Unless I'm snowplowing or carrying a load, I leave the shocks at "1" or the "very soft" setting. This tends to make the truck very floaty and it wallows over bumps, but I also don't feel bumps either.

At the bottom of the same dip, at the same speed, I nailed the brakes. Naturally, all four wheels locked up. But the truck only slid around 8 ft before stopping.

I'm going to have to try the same thing with my folks 2003 Buick LeSabre. It deals with the washboardy road very well, but I've never tried a panic stop on rough roads while driving their car. Just curious if all ABS cars react this way over certain road textures?

I do know one thing, if I had to drive over a washboardy 4km of gravel sideroad in my Prius everyday, I'd probably get rid of it. The suspension has absolutely no compliance for those sort of roads, either the car would shake itself to pieces or I'd need a bladder transplant.

 

<div class='quotetop'>QUOTE(jayman\";p=\"93268)</div>

I know on the one area of road where I can reproduce the problem on the prius almost every time I try, no othe ABS equipped car I've driven has that problem. They all just stop. My road isn't gravel tho, it's just regular pavement with undulations in it.

I'm glad I'm not the only one who sees this happen tho. Maybe this is a bigger problem than I thought at first.

 

The situation described does not sound like ABS or equipment malfunction to me; instead it sounds like something that might happen due to the (IMO) poorly-executed traction control on the Prius..

Does anyone know if the Prius traction control is active during braking?? If traction control cut in and disabled regen, the sudden loss of the regenerative load would certainly feel like losing the brakes, with the car lurching forward, especially since mechanical braking force is so low in non-emergency braking situations.. This is probably more of a problem with the HSD Prius with the new braking scheme where they've reduced the mechanical/increased the regenerative braking proportions, and as most Prius drivers learn quickly to use light braking to maximize regeneration, if the motor were suddenly switched out of the circuit, you'd have barely any contact with the brake pads, so if indeed this is what is happening, it's a very dangerous situation..

 

Ok, I'm somewhat confused:

to help discuss things, here are two diagrams.

Edit: diagrams were first posted by DanMan, who also did teh coloring.
Thanks DanMan!

The following one illustrates the break system of the 2005 Prius:
[Broken External Image]:http://www.tamug.edu/labb/LABBpeople/Markus/MINI/Prius brake map-sm.jpg

Full size img: Prius Brake Map

the second one illustrates the brake failsafe mode:
[Broken External Image]:http://www.tamug.edu/labb/LABBpeople/Markus/MINI/Brake Failsafe Mode-ct.jpg

there is essentially a direct hydraulic path from the master cylinder to the front wheel cylinders. This should ensure that if all goes haywire, you can still use the (front) brakes, although possibly (depending on degree of haywire) without power assist, and without any of the electronic features such as ABS and brake force distribution, and TC etc...

However, the diagrams also shows the master cylinder cut solenoid valves (1, and 2 or SMC 1 and 2). If things go haywire, these solenoids are OFF to keep the valves and the direct hydraulic path open.

However, could it be possible that one of several faults that might be at play here result in the computer shutting these valves (ON), to cut that path?

Basically, I would say that the only way for there to be NO braking power whatsoever, is for these SMC solenoids to be ON and these valves closed. This could happen if the computer shuts these inappropriately when other certain fault conditions exist.
Not sure if this makes sense, any comments?

M